1. Field of the Invention
The present invention relates to image processing apparatus and method for conducting color correction.
2. Related Background Art
In a prior art color image forming apparatus, a process of transferring a record image formed on a photoconductor drum by charging, light exposure and development is repeated for a plurality of colors to form a color image.
FIG. 38 shows a construction of a prior art color image forming apparatus.
In FIG. 38, a roller charger 102, a cleaning device 103, a developing device 104 and a transfer drum 105 are arranged around a photoconductor drum 101.
The developing device 104 is formed in a rotatable cylinder shape and four developing units 106a, 106b, 106c and 106d are provided therein and yellow, magenta, cyan and black toners are contained in the respective developing units 106a to 106d.
FIG. 39 shows a construction of the developing unit 106 (106a to 106d) and an application roller 107, a developing roller 108, a toner restriction member 109 and a toner container 110 are provided therein. As shown in FIG. 38, the developing roller 108 is exposed to the external from openings 111a to 111d of the developing units 106a to 106d. The application roller 107 is rotated to apply the toner contained in the toner container 110 to the developing roller 108, and necessary toribo is applied to the toner by the toner restriction member 109.
The transfer drum 105 comprises a resilient layer 113 on a metal cylinder 112 and a PVDF 114 on the resilient layer 113. A sheet feed roller 115, a gripper 116, a suction roller 117, a separation pawl 118, a fixing device 119, a cleaning device 120 and a discharging roller 121 are arranged around the transfer drum 105.
An optical unit 122 and a reflection mirror 123 are arranged above the photoconductor drum 101. The optical unit 122 comprises a laser diode, a laser driver, a rotating polygon mirror which is rotated at a high speed and a lens.
A sheet cassette 123 for containing transfer sheets, not shown, is arranged below the photoconductor drum 101.
The photoconductor drum 101 is driven in a direction a at a peripheral velocity of 100 min/sec by drive means, not shown. The photoconductor drum 101 has a photoconductor of organic photoconductor (OPC) material applied on an outer periphery of an aluminum cylinder having a diameter of 40 mm. A-Si, CdS or Se may be used as the OPC.
The material of the toner restriction member 109 is nylon when the toner is charged negatively, and silicone rubber when the toner is charged positively such that a material which is charged in the opposite polarity to that of the toner is used. A peripheral velocity of the application roller 107 is selected within a range of 1.0 to 2.0 times of the peripheral velocity of the photoconductor drum 101.
The transfer drum 105 has a resilient layer 113 of foamed urethane having a thickness of 2 mm wrapped on a metal cylinder 112 having a diameter of 156 mm and a PVDF 114 having a thickness of 100 .mu.m is wrapped thereon.
A prior art color image forming apparatus is disclosed in Laid-open Japanese Patent Application No. 50-50935. The developing device 104 is disclosed in Laid-open Japanese Patent Application No. 50-93437.
An operation in the above construction is now explained.
When the laser diode in the optical unit 122 is driven by a yellow image signal through the laser driver, the laser beam illuminates the photoconductor drum 101 through the reflection mirror 123.
An AC voltage of 1500 V peak-to-peak at a frequency of 1000 Hz is superimposed on a DC voltage of -700 V and the surface of the photoconductor drum 101 is uniformly charged to approximately -700 V. The illuminated area of the photoconductor drum 101 is at approximately -100 V and an electrostatic latent image is formed. As the photoconductor drum 101 is advance along the arrow a, the toner is deposited to the electrostatic latent image by the developing device 106a containing the yellow toner so that it is visualized.
On the other hand, a transfer sheet (not shown) fed from the sheet cassette 123 by the sheet feed roller 115 is held by a gripper 116 and then electrostatically sucked to the transfer drum 105 by the suction roller 117 to which the voltage is applied. The toner image on the photoconductor drum 101 is transferred to the transfer sheet sucked to the transfer drum 105 by the voltage applied to the transfer drum 105 from a power supply, not shown.
The above process is repeated for the respective colors of magenta, cyan and black to form multi-color toner images superimposed on the transfer sheet. The transfer sheet is scraped off the transfer drum 106 by the separation pawl 110 and then fused and fixed by the fixing device by heating and pressurizing to form a full color image.
The remaining non-transferred toners on the photoconductor drum 101 are cleaned by the cleaning device 103 including a fur brush and blade means. The photoconductor drum 101 is discharged by the discharging device and initialized. In the present example, the charging roller 102 is used for the charging of the photoconductor drum 101, and when the photoconductor drum 101 is discharged, the DC voltage is set to approximately 0 V while keeping the applied AC voltage as is.
The toners on the transfer drum 105 are also cleaned by the cleaning device 120 including the fur brush and the web. The transfer drum 105 is discharged by the discharging roller 121 and initialized.
The developing method is preferably a one-component developing system which does not require a complex construction such as an ATR or a screw and allows the use of a process cartridge system which enhances user maintenance. Of the one-component development system, a non-contact development system offers an advantage of simple construction.
The color image forming apparatus described above is for a contact development system in which the developing roller 108 and the photoconductor drum contact so that one of them must be resilient. In the non-contact developing system, however, those members may be an aluminum substrate and hence a cost merit is high.
Further, since the color toner renders the tonality of the output image better, it is desirable to use a toner of the sharp melt type which is instantly molten at certain fixing temperature. However, this type of toner often lowers a glass transition point and in the contact developing system, toner may be fused to one or both of the photoconductor drum 101 and the developing roller 108 by the abrasion of the photoconductor drum 101 and the developing roller 108. In order to prevent the fusing, it is desirable to use the non-contact developing system.
FIG. 40 shows a non-contact developing system in which four developing devices 202a, 202b, 202c and 202d are fixedly arranged around the photoconductor drum 101 and a color image may be formed without contact between the photoconductor drum 101 and the developing devices 202a to 202d.
When a color image is formed by using the above non-contact developing system, the inventors of the present invention found that a white clearance which should not be present was created between colors of the image formed by different adjacent colors and a white stripe was created as shown in FIG. 41. This is caused because the visualized image is formed narrower than the electrostatic latent image formed on the photoconductor drum when the latent image, for example, the image edge at which a drum surface potential abruptly changes is formed on the photoconductor drum and developed by the developing device. In monochromatic image formation, the narrowing of the image, even if it occurs to some extent, does not cause a problem because there is no adjacent color.
However, when a color image is formed under such a condition and a cyan band and a black band, for example, are adjacent in the image, the image in which the cyan band and the black band should appear adjacently includes a clearance between the cyan band and the black band in the final image on the transfer sheet because the visualized cyan image and the visualized black image are narrowed respectively.
As shown in FIG. 42, such narrowing of the image is a phenomenon caused by the narrowing of the edge as shown by the visualized area because the electric field is wrapped at the edge (shown as the latent image area) of the electrostatic latent image formed on the photoconductor drum, and the affect appears more prominently in the non-contact developing system.
The present invention is intended to solve the above problems and it is an object of the present invention to provide a color image forming apparatus which eliminates the narrowing of the image.
In order to achieve the above object, the present invention provides an image processing apparatus for color-correcting image data for outputting to an image forming means, comprising input means for inputting image data; discrimination means for determining whether or not the image data is of a predetermined color; color correction means for color-correcting the image data even if the image data is lower than a predetermined level in such a manner that a light emitting device in the image forming means emits a light at a small light emission level; and output means for outputting the color-corrected image data or image data generally representing white to the image forming means based on the result of the determination.
It is other object of the present invention to prevent the narrowing of the image without causing a fogging phenomenon in a non-print area.
In order to achieve the above object, the present invention provides an image processing apparatus comprising input means for inputting color image data; edge detection means for detecting an edge based on the input color image data; and control means for controlling a light emitting device in an image forming unit to emit light at a small emission level for pixels near the edge.
Other objects and features of the present invention will be apparent from the following description of the embodiments and the accompanying drawings.